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BALLOON PHOTOGRAPHS.

NOTHER application of photography and a new use for balloons has been opened by M. Paul Desmarets. He makes a hole in the bottom of the car and there places a camera; and thus, by instantaneous process, obtains a map-like portrait of the country below, which may be printed by the autotype or some other of the many processes now in operation. These photographs admit of considerable magnifying of details, which greatly adds to their interest and value. M. Desmarets's pictures were taken over Rouen, and arrangements are now in progress for the systematic photography of Paris from above. It remains to be seen whether practically useful maps may be thus produced, and to what extent existing maps may be corrected by these interesting sun-pictures. The most curious element of the invention is that it should be a novelty, that so obvious and simple an idea should not have been carried out long ago.

Commander Cheyne must not omit the hole in the bottom of his balloon car, and must carry suitable photographic apparatus. He should take lessons in photography forthwith. The copyright of midnight sun-pictures of the Pole, if well worked, might pay the expenses of his expedition.

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GEOLOGICAL CONVULSIONS.

E all know that land and sea have changed places, and that even the tops of high mountains were once under the sea. I have myself found fossil remains of marine shells on the summit of Mount Pilatus, which is more than 7,000 feet above the present sea level; but these belong to a geological era long passed away, and their inhabitants were probably animals that dwelt in shallow waters near the shore.

Mr. Gwyn Jefferys has examined some fossil shells found in Calabria and Sicily at heights of more than 2,000 feet above the sea level, and finds them of the same species as others that are now living at depths of not less than between 9,000 and 10,000 feet below its surface, and dredged up during the expeditions of the Lightning and the Porcupine. If the inference that they cannot, as well as do not, live at less depths is correct, their existence in this position indicates an upheaval of eleven to twelve thousand feet within a period which, geologically speaking, is but recent. The probability of this great. change is increased by the fact that the whole region between Vesuvius and Etna is still a literal hot-bed of volcanic activity, and

that even monuments of human handiwork thereabouts have been sunk below the sea and afterwards raised above it, as shown in the celebrated instance of the temple of Jupiter Serapis at Puzzuoli, the remaining columns of which bear the perforations of marine stoneborers up to a height of 23 feet above the present sea level, marking this as the depth of their submergence and re-elevation, not merely since the temple was built, but since it became a ruin. This is shown by the fallen columns, which are not perforated in the same manner as those which stand upright.

EYE-MEMORY.

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OOK steadily at a bright object, keep the eyes immovably on it for a short time, and then close them. An image of the object remains; it becomes, in fact, visible to the closed eyes. The vividness and duration of such impressions vary considerably with different individuals, and the power of retaining them may be cultivated. Besides this sort of retinal image thus impressed, there is another kind of visual image that may be obtained by an effort of memory. Certain adepts at mental arithmetic use the "mind's eye" as a substitute for slate and pencil by holding in visual memory pictures of the figures upon which they are operating, and those of their results.

In my youthful days I was acquainted with an eccentric old man who then lived at Kilburn Priory, where he surrounded himself with curious old furniture reputed to have originally belonged to Cardinal Wolsey, and which, as I was told, he bequeathed to the Queen at his death. He was the then celebrated, but now forgotten, "Memory Thompson," who in his early days was a town traveller (for a brewery, if I remember rightly), and who trained himself to the performance of wonderful feats of eye-memory. He could close his eyes and picture within himself a panorama of Oxford Street and other parts of London, in which picture every inscription over every shop was so perfect and reliable that he could describe and certify to the names and occupations of the shopkeeping inhabitants of all the houses of these streets at certain dates, when Post-Office Directories were not as they now are.

Although Memory Thompson is forgotten, his special faculty is just now receiving some attention, and it is proposed to specially cultivate it in elementary schools by placing objects before the pupils for a given time, then taking them away and requiring the pupil to draw them. That such a faculty exists and may be of great service is

unquestionable. Systematic efforts to educate it, if successful, will do good service to the rising generation; and, even should the proposed training affords smaller results than its projectors anticipate, the experiments, if carefully made and registered, cannot fail to improve our knowledge of mental physiology.

We are told that the "second-sight" trick practised so successfully by Houdin and his son was done by cultivating this faculty. I suspect, however, that Houdin's confidential accounts of his training of himself and son in acquiring thus the art of visual memory were strictly professional. The second-sight trick, as I have seen it done, is performed quite differently, the objects described never having been seen at all by the person describing them, but being under the eye of the questioner. It depends on a very skilful framing of questions which convey information through a series of predetermined signals, demanding months and even years of continual practice to carry out. When a conjuror takes you into his confidence and explains the principle upon which one of his best tricks is done, you may take it for granted that he is practising upon you the fundamental principle of all his tricks, viz. that of misdirecting your attention. If he talks about the machinery of his automaton, allows you to discover that he was once apprenticed to a watchmaker, and carefully winds up the machinery in the box under the figure before it begins to perform, you may safely conclude that there is no machinery there beyond what is necessary to produce the ostentatious clicking that accompanies the winding.

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THE UNDER CRUST OF THE EARTH.

VERYBODY now knows that the legendary apple which startled Sir Isaac Newton was brought down by gravitation, and also that the velocity of its fall was proportionate to the mass of the earth; but there are many educated people who would be puzzled to tell how a body may be continually falling towards the earth for days and months and years, without striking or getting any nearer to it. This, however, is done by the moon, which would go straight on and leave the earth altogether but for the earth's gravitation. Newton understood this, and, by measuring the continuous and earthward bending of the moon's path, determined the rate of its continuous fall towards the earth, thereby verifying his hypotheses; and is said to have swooned when he found that it exactly matched his calculations.

This suggests another problem. Can we construct an artificial

satellite that shall be continually falling towards the earth without touching it, and at the same time continue within our reach down here, as we stand upon the surface of the earth? This appears paradoxical, but may be done, and is done most easily. A pendulum is such a body, with the great advantage of being capable of measuring and recording its own velocity of fall by the aid of the well-known machinery of a clock.

By such a device Airy ascertained the difference between the gravitating power of the earth on its surface when its whole mass was pulling the pendulum downwards, and that which it exerted at the bottom of a deep coal-pit, when the portion above the pendulum was pulling it upwards, and the quantity below was by so much diminished.

Mr. Faye has lately been working with similar tools, and collecting the data of other pendulum workers, with some interesting results. He finds that while certain small mountains, such as Schiehallion, Arthur's Seat, &c., add the action of their masses to the gravitating work of the earth upon the pendulum, other vastly greater masses, such as the Himalayas, do not; and that the force of gravitation is even less upon some elevated continents than over the sea. The mountains act as though undermined by great cavities. Mr. Faye does not, however, suppose this to be the case, but suggests a far more probable explanation, viz. that below the ocean the specific gravity or density of the crust of the earth is greater than below the great continents, and that this is due to difference of temperature.

We know, as a positive fact, that in sinking mines, artesian wells, &c., the temperature increases as we descend, after the first hundred feet or thereabouts is passed; and this increase has been attributed to the internal heat of the earth, which can but very slowly escape through the ill-conducting solid crust.

But at the bottom of the ocean the water is icy cold at depths so great that we should reach the boiling-point of water, or still higher temperatures, if we could sink so far below the land surface. Thus the mean density of water and rock under the deep Atlantic may be greater than the mean density of the continuous solid under a continent.

If this is correct, there must be a continual squeezing downwards over great oceanic areas, and a squeezing upwards under continents, which squeezing will operate in the way of upheaval wherever the material is sufficiently plastic. This, as may be easily understood, opens up a wide field for geological speculation, and has an important bearing on Mallet's theory of volcanoes and earthquakes, and of mountain formation.

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THE IMMATERIALITY OF MATTER.

R. CROOKES, to whom the world is indebted for that marvellous little instrument the radiometer, for the curious and important researches that led to its construction, and for subsequent investigation of strange molecular mysteries, is not satisfied with having brilliantly displayed some of the properties of what he denominates the "ultra-gaseous" condition of matter, but has suggested a new version of material existence. The modern view of the constitution of matter is that it is made up of molecules; that heat is, as Dr. Tyndall expresses it, "a mode of motion," i.e. motion of these molecules, which, when communicated to our organs of sense, produce the feeling of temperature. Mr. Crookes goes farther, and maintains that what we call matter "is nothing more than the effects upon our senses of the movements of molecules." According to the generally accepted mathematical view of the constitution of matter, these molecules are inconceivably small, and the interspaces through which they swing, or vibrate, or fly, or gyrate, although utterly invisible, are vastly larger than the molecules themselves. Mr. Crookes adds to this conception that "the space covered by the motion of the molecules has no more right to be called matter than the air traversed by a rifle bullet has to be called lead. From this point of view, then, matter is but a mode of motion; at the absolute zero of temperature the intermolecular motion would stop, and, although something retaining the properties of inertia and weight would remain, matter, as we know it, would cease to exist.”

Would Mr. Crookes still maintain this view of the nature of matter if a cannon-ball or a 56-lb. weight were cooled down to the absolute zero of temperature and dropped upon his toe, that particular toe bearing a sensitive corn? Would he be thereby convinced that these residual "properties of inertia and weight" are sufficient to constitute "matter as we know it"? I think he would. On my own part, I would give up the argument at once without trying the experiment.

I have always been a sceptic in respect to the ultimate molecular or atomic constitution of matter, and have watched the researches of Mr. Crookes with considerable interest, believing that ere long they will refute the complexities of modern mathematical speculations concerning the dancing of molecules, and lead to more simple and natural conceptions. If Mr. Crookes proceeds far enough in the same direction as that in which he is now moving to supply us with a complete reductio ad absurdum of the prevailing mathematical